This invention relates to apparatus for generating coded characters and, more particularly, to such apparatus wherein a bi-phase coded character is generated having a constant relation between the phases at the beginning and end of each coded character regardless of the number of "1"s and "0"s included in that character. In accordance with a specific aspect of this invention, an SMPTE time code character is generated having a level transition at the start thereof which is of the same phase, or polarity, as the level transition at the end thereof.
With the advent of video recording and playback devices, such as the video tape recorder (VTR), various situations may arise wherein a recorded medium, such as recorded video tape, should be edited such that certain information thereon is replaced by other, desired information, or that certain additional information is recorded immediately following a previously recorded segment. The former editing operation is known as an insert edit wherein, for example, one or more frames of video signals which are recorded on one video tape are replaced by an equal number of frames of video signals which had been recorded on another video tape. The latter edit operation is known as an assemble edit wherein one of more frames of video signals which had been recorded on another video tape are transferred, or re-recorded, onto the main video tape, whereby an entire program may be "assembled".
As may be appreciated, in order to carry out an insert edit or an assemble edit operation, that is, an operation wherein video signals which had been recorded on a secondary tape are transferred, or re-recorded, onto a primary tape, two separate VTR's are needed for controlling the respective playback and recording operations of the secondary and primary tapes. The secondary VTR must be operated to ascertain those frames containing the video information which must be transferred to the primary tape and the primary VTR must be controlled so as to ascertain the precise location on the primary tape at which the secondary information is to be recorded. Furthermore, both VTR's must be operated in synchronism with each other.
In order to facilitate such editing operations, and particularly, to identify the specific frames of video signals which are to be edited, a standardized code has been developed, known as the SMPTE time code. This SMPTE time code is a serial code formed of a predetermined number of binary bits which are recorded as phase-modulated, or bi-phase signals. In accordance with the present standard, the SMPTE time code contains eighty bi-phase bits which represent, in BCD format, an indication of the time at which each frame of video signals is recorded, a frame count and optional binary word information. This time code also includes a sixteen bit synchronizing word and a number of so-called binary groups which may be used either for future information or as the user of the video tape so desires. A more complete description of this SMPTE time code is described in "Standardization for Time and Control Code for Video Tape and Audio Recorders" by E. K. Dahlin, December 1970, Journal of the SMPTE, Volume 79, page 1086. Another proposal for a standardized time code for video recording is the EBU time code.
In the bi-phase representation of binary bits, the beginning of each bit period is marked by a level transition, either a positive transition from a lower to a higher level or a negative transition. A binary "1" is represented by yet another level transition during the bit period, while a binary "0" is represented by the absence of any additional bit transitions throughout the bit period. In recording the SMPTE time code on video tape, the bi-phase signals are recorded in seriatum along a longitudinal edge of the tape such that the beginning of an SMPTE character starts at the beginning of the frame of video signals and ends at the end of that frame of signals. Thus, the end of one SMPTE character is coincident with the beginning of the next following SMPTE character. Accordingly, it is important that the phase of the beginning of an SMPTE character, that is, the direction of the level transition at the start of the SMPTE character, be consistent, or compatible, with the phase at the end of the preceding SMPTE character. This means that if the preceding SMPTE character ends with a negative-going transition, the next following SMPTE character should start with a negative-going transition.
In the insert edit operation wherein video signals which has been recorded on the primary tape are replaced by video signals which are played back from the secondary tape, that is, the secondary video signals are "inserted" between two existing segments on the primary tape, the SMPTE time code characters which had been recorded for the particular segment on the primary tape which is being replaced normally remain on the primary tape. Thus, in the insert edit operation, there is no change in the recorded SMPTE characters and, therefore, there is no problem in making sure that the phase at the end of a preceding SMPTE character is compatible with the phase at the beginning of the next following SMPTE character. However, in the assemble edit operation, a new SMPTE time code is recorded together with the new video signals which are recorded from the secondary video tape. Since the new SMPTE time code characters are not recorded during the same operation that the preceding SMPTE time code characters were recorded, it is possible that the phase at the beginning of a new character may not be compatible with the phase at the end of the preceding, previously recorded character. It has been thought that this problem can be solved by, for example, reading the next-to-last SMPTE time code character which is recorded on the primary tape just prior to the assemble edit location thereon and then interpolate the information represented by the read character such that the content of the last character can be ascertained, and thus the phase at the end of that last character will be known. Once this phase is known, the new SMPTE time code characters can be generated so as to be compatible therewith. However, this assumes that the last few frames of the previously recorded video signals on the primary tape had been recorded in succession. Thus, the frame count contained in the last character will be one greater than the frame count included in the preceding character, and so on. This technique is not successful if successive frames of video signals are assembled on a frame-by-frame basis, such as for assembling an animated video picture. In that event, the content of the last-recorded SMPTE time code character will not merely be one frame greater than the SMPTE time code character which precedes it.